Planarization of layers over semiconductor substrates is becoming increasingly more important as step coverage issues play a more predominant role in the manufacturing of semiconductor devices. Endpoint detection of polishing steps is becoming an important area within the field of polishing.
Polishing of semiconductor substrates has been performed since the time when semiconductor wafers were first made. A number of optical and other methods exist for determining when polishing endpoint has occurred. These methods include acoustical wave generation and detection, thermal imaging, friction sensing, impedance or capacitance measurements, monitoring the current of the motor used to rotate a wafer against a polishing pad, stylus profilometry, phase shift interferometry, light scattering analysis, scanning tunneling microscopy, atomic force microscopy, and three dimensional optical profiling.
Generally, monitoring substrates during polishing is more difficult when the substrate has been previously patterned compared to an unpatterned substrate. For an unpatterned substrate, such as a silicon wafer or the like, no underlying topography exists. Therefore, whatever topography is seen by the monitor is also present on the surface and is not a topography of an underlying surface. A similar result may hold for a single layer over an unpatterned substrate because the unpatterned substrate may be assumed to be substantially flat.
Monitoring the polishing of a layer over a previously patterned semiconductor substrate is harder. For many optical methods, distinguishing between the topography changes of the underlying patterned semiconductor substrate and the topography changes in the layer being polished may be nearly impossible to discern. Optical methods also suffer because many layers, such as metallic layers, are opaque to light. An anti-reflective coating (ARC), particularly for metal layers, makes optical thickness measurement difficult because an ARC on a metal layer does not give reliable measurements. Stacked oxides (one oxide on another oxide) also has optical measurement problems because the readings for the thickness typically include a cumulative error, which can be a problem with process control at a specific step. Mechanical methods, such as dragging a stylus across a surface of the layer may be used, but dragging a stylus across the surface of the layer may damage the layer.
Analysis of polished layers over patterned semiconductor substrates is typically performed by drying the substrates and moving the substrates to an analyzer. After polishing and before analysis, many polishing processes require a "light-duty" polishing step to remove slurry particles from the surface of the layer before it is dried. Otherwise, some of the slurry particles may become dried on the surface of the layer and may become very difficult to remove. The drying is required because many types of analytical equipment cannot accurately analyze the layer if it is wet or has particles on it. Moving the substrate between the polisher and the analytical equipment is typically performed by operators. The additional handling adds to costs and the additional handling typically causes yields to lower.